By virtue of their low cowl angles, mixed compression inlets for supersonic aircraft (e.g., missiles) have lower drag than all-external compression inlets. However, they also have lower pressure recovery than external compression inlets due to the fact that the internal area contraction, or supersonic diffuser, within the inlet is limited by a supersonic flow "starting" phenomenon. "Starting" can be defined as a stable condition in which the flow is supersonic at the cowl inlet lip and the terminal normal shock wave is located downstream of the inlet throat (i.e., the minimum cross-sectional area of the inlet duct). Starting of mixed compression inlets with internal area contractions greater than the limit can be implemented by varying the internal geometry of the duct with remote actuators. This provides an increase in pressure recovery, but this approach is too costly for many missiles, particularly tactical missiles.
Aside from the starting problem, supersonic inlets for missiles generally have covers for reducing the overall weapon stores drag when carried on an aircraft and for reducing missile drag during the rocket-boost phase of the the missile. These covers are normally ejected just prior to ramjet ignition, thus posing a hazard to the missile and/or launch aircraft.